Strangers can keep their hands-off my children, my grandchildren, and my wife. Just because a stranger is in proximity to these people does not in any way imply the stranger has the ability to touch them. Same goes with a vehicle, it's not a 'flesh & blood' member of the family, but it is a possession you have no right to touch, just because you happen to be in physical proximity to it. It's not your resting post, your stool, or a place to throw your trash (how many of us have found someone's garbage in the bed of their truck?).
Legally, now one in twenty residents of Florida have concealed carry permits. If you think that because no one's around the vehicle, that gives you the ability to open the door and rummage through its contents, looking for a handgun, rifle or ammo (the NRA sticker on the window), or anything of value you find, well, you are taking a big chance when I show up after getting a silent page and find you, the thief, inside.
Sitting on a vehicle or leaning up against it is just a stupid, careless, and aggressive thing to do, it's inconsiderate, and as far as I know, when I see someone leaning against my vehicle the reason they are there is to victimize me in some way.
If you say, "hey, I wuz jes' hangin,' chill!" that doesn't turn-on any 'warm & fuzzy' kumbaya for me. I still perceive you as a threat to my safety and the safety of my family with me, until proven otherwise, and I'm not staying around long-enough to wait for an "opportunity development" moment to occur, back the &^$$#-away from my truck, or car, or motorcycle, or my family. My family's safety is more-important to me than your need to rest your sorry *** on or against my vehicle.